The studies suggesting green tea can have a slight beneficial effect with regards to certain types of cancer are inconclusive as the data is of very low quality and inferences are unsafe. While anti-oxidants are a great marketing ploy, anyone familiar with the Haber-Weiss reaction knows that the direction of the reaction (Fe2 <> Fe3) is acutely sensitive to concentration; furthermore, if we mute the signals from mitochondria by means of excessive anti-oxidant intake we ensure we have to carry on with worn-out mitochondria instead of inducing apoptosis and thereafter replacing them with new and more efficient cells. The one thing that is reasonably certain, however, is that a mixture of small amounts of caffeine taken with slightly greater amounts of L-theanine can induce sustained calm concentration. Studies suggest 25mg - 40mg of caffeine with 40mg - 60mg L-theanine is optimal for most people, unless they are so addicted to highly caffeinated beverages that their physiology can no longer respond adequately to more beneficial levels.